[SPARK-15704][SQL] add a test case in DatasetAggregatorSuite for regression testing

## What changes were proposed in this pull request?

This change fixes a crash in TungstenAggregate while executing "Dataset complex Aggregator" test case due to IndexOutOfBoundsException.

jira entry for detail: https://issues.apache.org/jira/browse/SPARK-15704

## How was this patch tested?
Using existing unit tests (including DatasetBenchmark)

Author: Hiroshi Inoue <inouehrs@jp.ibm.com>

Closes #13446 from inouehrs/fix_aggregate.